The ever-increasing demand for broadband communication systems has led to optical-transmission systems based on optical waveguides such as fiber optics and optical processing elements for use in these systems. Generally, in high-performance communication systems, photons continue to supplant electrons as messengers.
Significant effort has been spent towards optical integrated circuits with high complexity and advanced functionality. As is described in Driessen et al., Proc. of SPIE Vol. 5956, 2005, which is hereby incorporated by reference herein, optical “microresonators” can be considered as promising building blocks for filtering, amplification, switching, and sensing. Active functions can be obtained by monolithic integration or a hybrid approach using materials with thermo-optic, electro-optic, and optoelectronic properties and materials with optical gain. Driessen does point out that there is a need for “better design tools for single devices as well [as] systems” among other needs.
In a common configuration in microresonator-based sensors, a microresonator is placed in close proximity to an optical waveguide such as an optical fiber whose geometry has been specifically tailored—for example, tapered or etched to a size of 1-5 microns. The tapering modifications to the waveguide result in a substantial optical field outside the waveguide, so that light can couple into the microresonator and excite its eigenmodes. These eigenmodes may be of various types, depending upon the resonant cavity geometry.
For spherical and disk cavities, the modes of interest for sensing applications are usually the so-called “whispering gallery modes” (WGMs), which are traveling waves confined close to the surface of the cavity. Since the WGMs are confined near the surface, they are well-suited to coupling with optical fibers, optical waveguides or analytes placed on or near the surface.
For ring cavities based on single-mode waveguides, the modes are those of the single-transverse-mode channel waveguide, under the constraint that the round-trip path traversed corresponds to an integral number of wavelengths. Other cavity geometries, such as Fabry-Perot resonators using single-mode waveguides with Bragg grating reflectors, or multimode rectangular cavities, have familiar standing-wave resonances as their eigenmodes.
In U.S. Pat. No. 7,215,848, Tan et al. disclose an optical isolator for coupling light from a first optical waveguide to a second optical waveguide. The optical isolator utilizes a microresonator coupled to the first and second optical waveguides.
MacFarlane and co-workers describe an active lattice filter structure for use in optical signal processing (Kannan et al., IEEE Journal of Lightwave Technology, Vol. 24, No. 71, 2006; Hunt et al., EURASIP Journal on Applied Signal Processing, 2005:10). In these filters, certain filter parameters (Ki and ti according to the nomenclature of the present invention) were fixed after the filter was constructed, leaving gain as the only available vehicle for programming the filter's response. It would be beneficial if Ki and ti could be made to be tunable.
In the art, it has been typically necessary to resonantly couple multiple microdisks placed in close proximity to each other, so as to obtain “flattened” passbands from the intrinsic Lorentzian passbands of the microdisks or Fabry-Perot resonators (see, for example, Little et al., IEEE J. of Lightwave Technology, Vol. 15, No. 6, 1997).
Also, in Jinguji, IEEE J. of Lightwave Technology, Vol. 14, No. 8, 1996, as well as Madsen, IEEE Photonic Technology Letters, Vol. 10, No. 8, 1998, passive filters are constructed from micro-rings that are not programmable. Because variable coupling between the waveguide-interconnects and micro-rings is not available in these structures, neither two-dimensional connectivity nor passband reconfiguration can be readily achieved in the filters described by Jinguji and Madsen.
In view of the shortcomings in the art, there is a need for methods that provide for reversible adjustment of optical-filter parameters, thereby addressing the aforementioned call for better design tools pertaining to the use of microresonators. What is needed is a heterogeneously integrated filter structure that is suitable for controlling the transfer of optical power between the microresonator and the waveguide in an efficient (low loss) fashion, and for tuning of resonance frequencies.
Improvements should also set forth the manner of making and using the filter in practical devices and systems, such as those that employ channelization. “Channelization” refers to the filtering or division of a broadband signal (such as radio frequency) into narrower frequency-bands, or channels.
In defense-related systems, one can encounter threats over a broad spectrum of radio frequencies. The systems need to cover the entire spectrum, with sufficient selectivity to separate simultaneously received signals that are closely spaced in frequency. These requirements can be met through channelization.
Therefore, in view of the above-described shortcomings, there is a further need in the art for filtering methods and filter structures that can be utilized for narrowband channelization of radio frequency signals that have been modulated onto optical carriers. These filtering methods should enable the filtering of radio-frequency (RF) signals in the optical domain, i.e. filtering the RF signals without demodulating it from the optical carrier.